Vacuum-heat-insulated cooking utensil and method of manufacturing same

ABSTRACT

Disclosed is a vacuum-heat-insulated cooking utensil and a method of manufacturing the same. Inner and outer containers are prepared each having a tubular side wall and an end wall closing the end of the side wall. The inner container is positioned in the outer container with the respective bottoms being in contact with each other through a first brazing material, with the other ends of the side walls fitted to each other with a gap which constitutes a vacuum sealing section, and with an annular space being defined between the side walls. A second brazing material having a melting point higher than that of the first brazing material is positioned at the vacuum sealing section. An assembly of the containers and the brazing materials is placed in a vacuum furnace. The furnace is evacuated to evacuate the annular space. Temperature within the furnace is raised above the melting point of the second brazing material to melt the first and second brazing materials. The temperature within the furnace is then lowered to a level lower than a solidifying point of the second brazing material but higher than the melting point of the first brazing material to solidify the second brazing material to seal the vacuum sealing section. Subsequently, pressure within the furnace is raised to a level approximate to the atmospheric pressure. The temperature within the furnace is lowered to solidify the first brazing material to bring the end walls of the respective containers into intimate contact with each other to form a heat receiving bottom of the cooking utensil.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a vacuum-heat-insulated utensil capableof maintaining temperature of foods after heated and cooked constant,while the foods are kept contained in the cooking utensil.

2. Related Art Statement

In recent years, various kinds of vacuum-heat-insulated cooking utensilssuch as a pan, cookpot, kettle and the like have been proposed which arecapable of maintaining temperature of foods after heated and cookedconstant, in such condition that the foods are contained in the cookingutensils, without the necessity of emptying of the cooking utensils intoanother heat-insulated container. Such a vacuum-heat-insulated cookingutensil generally comprises a container body having a double-walled,vacuum-heat-insulating side wall structure and a heat receiving bottom.

There is known a vacuum-heat-insulated cooking utensil of the type whichis manufactured by connecting first ends of a pair of cylindrical sidewall members to a disc-shaped metal plate or a heat receiving member andby connecting the other ends of the side wall members together with anannular member to thereby provide an annular space between the side wallmembers to be evacuated. There is also known a vacuum-heat-insulatedcooking utensil of the type which is manufactured by connecting aseparate tubular wall member to a side wall of a cup-shaped container toform the double walled, heat-insulating side wall structure.

However, the above-described conventional vacuum-heat-insulated cookingutensil having the inner and outer side walls integrally connected tothe bottom plate has suffered from such a disadvantage that in view ofthe complexity in construction, the manufacturing per se of such cookingutensil would become difficult and would increase the cost. On the otherhand, the cooking utensil in which the separate wall member is connectedto the side wall of the container has suffered from such disadvantagesthat the cooking utensil would be deteriorated in design, and the jointbetween the separate wall member and the side wall which is to beairtight would be complicated in structure and the welding of the jointwould become troublesome.

As described above, the conventional vacuum-heat-insulated cookingutensils have the problem in construction or manufacturing manner andhave not yet been put into practical use, and it has been desired tosolve the problems.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a cookingutensil having a vacuum-heat-insulating wall structure which is simplein construction and can be manufactured at relatively low costs.

It is another object of the present invention to provide a method ofmanufacturing such a vaccum-heat-insulated cooking utensil.

According to one aspect of the present invention, there is provided avacuum-heat-insulated cooking utensil comprising inner and outercontainers each made of metal and having a tubular side wall and an endwall closing one end of the side wall, the side wall of the outercontainer having an inner diameter greater than an outer diameter of theside wall of the inner container, the end wall of the inner containerbeing brazed at an outer surface thereof to an inner surface of the endwall of the outer container in such a manner that the inner container isdisposed in coaxial relation to the outer container to form an annularspace between the side walls of the inner and outer containers, theother ends of the inner and outer containers being hermetically brazedto each other to close the annular space, the closed annular space beingevacuated to provide a heat-insulating space.

According to another aspect of the invention, there is provided a methodof manufacturing the vacuum-heat-insulated cooking utensil comprisingthe steps of: providing inner and outer containers each made of metaland having a tubular side wall and an end wall closing one end of theside wall, the side wall of the outer container having an inner diametergreater than an outer diameter of the side wall of the inner container;placing the inner container in the outer container in coaxial relationto provide a container assembly, with a first brazing materialsandwiched between the end walls of the inner and outer containers andwith a second brazing material disposed between the side walls adjacentto the other ends thereof, an annular space being formed between theside walls, the second brazing material having a melting point higherthan that of the first brazing material, the other ends of the sidewalls being spaced from each other a predetermined distance to providean evacuation gap; placing the container assembly in a vacuum chamberwith an axis thereof disposed vertically and with the other ends of theside walls disposed at a level lower than the end walls; evacuating thevacuum chamber to evacuate the annular space through the evacuation gap;subsequently heating the container assembly in the vacuum chamber to atemperature to melt the first and second brazing materials, so that themolten first brazing material is filled in a space formed between theend walls and that the molten second brazing material flows into theevacuation gap; subsequently cooling the vacuum chamber to a temperatureto solidify the molten second brazing material to close the evacuationgap; and subsequently increasing pressure in the vacuum chamber to sucha level to move the end walls toward each other and cooling the vacuumchamber to a temperature to solidify the molten first brazing material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cross-sectional front elevational view showingcomponents to be assembled into a vacuum-heat-insulated cooking utensilaccording to the present invention;

FIG. 2 is a partially cross-sectional front elevational view showing theassembly of the components shown in FIG. 1;

FIG. 3 is a cross-sectional front elevational view of a vacuum furnacein which the assembly shown in FIG. 2 is placed.

FIG. 4 is a graph showing changes in temperature and pressure within avacuum chamber of the vacuum furnace of FIG. 3 when the manufacturingmethod in accordance with the present invention is carried out;

FIG. 5 is a partially cross-sectional front elevational view showing thevacuum-heat-insulated cooking utensil manufactured by the methodaccording to the present invention;

FIG. 6 is a fragmentary enlarged cross-sectional view showing a portionencircled by the character V in FIG. 5; and

FIG. 7 is a fragmentary enlarged cross-sectional view showing a portionencircled by the character VI in FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows components to be prepared to manufacture avacuum-heat-insulated cooking utensil in accordance with the presentinvention. The components include an inner container, generallydesignated by the reference character 1, which is made of a metal suchas stainless steel or the like and has a tubular or cylindrical sidewall 2 and an end wall or bottom 3 closing one end of the side wall 2.The side wall 2 has the other end enlarged in diameter to form anenlarged diameter portion 4 of a predetermined depth which defines amouth of the inner container 1. An annular flange (first flange),generally designated by the reference numeral 5, extends radiallyoutwardly a predetermined distance from a free end of the enlargeddiameter portion 4, and has an outer peripheral portion 6 extendingdownwardly to define an annular recess 7.

An outer container, generally designated by the reference numeral 8, ismade of a metal such as stainless steel or the like, and has a tubularor cylindrical side wall 9 and an end wall or bottom 11 closing one endof the cylindrical side wall 9. An annular flange 12 (second flange)extends radially outwardly from the other end of the cylindrical sidewall 9, which other end defines a mouth of the outer container 8. Theside wall 9 has an inner diameter which allows the enlarged diameterportion 4 of the inner container 1 to be fitted with a small gap intothe side wall 9. In other words, the inner diameter of the side wall 9is slightly greater than the outer diameter of the enlarged diameterportion 4. The flange 12 has a radial extent capable of being looselyfitted in the recess 7. The outer container 8 has a depth substantiallyequal to that of the inner container 1.

A first brazing material 13 is prepared to braze the end walls 3 and 11of the respective inner and outer containers 1 and 8 to each other. Thefirst brazing material 13 consists of a silver brazing filler metal orthe like and is in the form of a thin disc having a diametersubstantially equal to the diameter of the end wall 3 of the innercontainer 1. A second brazing material 14 is prepared to seal in agas-tight manner an evacuation gap or a vacuum sealing section 15 (cf.FIG. 2) at the mouths of the respective inner and outer containers 1 and8. The second brazing material 14 consists of a silver brazing fillermetal or the like having a melting point higher than that of the firstbrazing material 13, and is in the form of a ring having an outerdiameter substantially equal to the inner diameter of the side wall 9 ofthe outer container 8. Preferably, the first brazing material 13 has agreater thermal conductivity than the second brazing material 14.

An example of a manufacturing method of the vacuum-heat-insulatedcooking utensil, using the above-described components, will now bedescribed with reference to FIGS. 2 to 4.

The inner container 1 is first placed on a surface (not shown) with themouth thereof facing downwardly, as shown in FIG. 2. The first brazingmaterial 13 is then placed on the end wall 3 of the inner container 1,and the second brazing material 14 in the form of a ring is placed onthe enlarged diameter portion 4 of the side wall 2 with the remainingportion of the side wall 2 extending therethrough. The outer container 8is then put on the inner container 1 from the above such that the flange12 of the outer container 8 is fitted into the recess 7 defined by theexpanded portion 4 and the flange 5 to fix the outer container 8 to theinner container 1 and to define an annular space 16 between the sidewalls 2 and 9 of the respective inner and outer containers 1 and 8. Inthis case, the first brazing material 13 is sandwiched between thebottoms 3 and 11, so that a small gap is formed between the flange 12and the bottom of the recess 7. Also, an annular gap is formed betweenthe outer surface of the enlarged diameter portion 4 and the innersurface of the side wall 9 since the inner diameter of the side wall 9is slightly greater than the outer diameter of the enlarged diameterportion 4 of the side wall 2, and the second brazing material 14 liesalong an upper end of the annular gap. The annular space 16 thereforecommunicates with the exterior of the thus assembled inner and outercontainers 1 and 8 through the annular gap formed between the enlargeddiameter portion 4 and the side wall 9 and through the gap existingbetween the flange 12 and the bottom of the recess 7. These two gapsconstitute the evacuation gap or the vacuum sealing section 15.

Subsequently, the assembly of the inner and outer containers 1 and 8with the first and second brazing materials 13 and 14 being located intheir respective positions is put in a chamber 100 of a vacuum furnace101, as shown in FIG. 3, through a first air lock chamber 100a thereof.The vacuum furnace 101 has a vertically movable table 102 on which theassembly is placed, an electric heater 103 of a cylindrical shape, and asecond air lock chamber 100b through which the processed assembly istaken out. The table 102 is moved upwardly to the position where theassembly is entirely surrounded by the heater 103, when it is desired toheat the assembly. The vacuum furnace 101 also comprises a vacuum pump105, a gas source 106 of an inert gas such as nitrogen, argon or thelike, control valves 107 to 110, and a control circuit (not shown) whichcontrols the vacuum pump 105, the control valves 107 to 110 and theelectric heater 103.

FIG. 4 shows changes in pressure and temperature, to which the assemblywithin the vacuum furnace 101 is subjected, over time. As indicated by abroken line in FIG. 4, the chamber 100 is first evacuated to decreasethe pressure therein to a level P1. It is desirable that the pressure P1is below 1×10⁻² Torr. This allows the annular space 16 to be evacuatedthrough the vacuum sealing section 15 to decrease the pressure withinthe space 16 to a level equal to the pressure P1.

After the interior of the chamber 100 has been sufficiently evacuated,the table 102 is lifted to move the assembly into the heater 103 andthen the heater 103 is energized to raise the temperature of theassembly to a level T1, as indicated by a solid line in FIG. 4, which ishigher than the melting point of the second brazing material 14, whilemaintaining the pressure within the chamber 100 at the pressure P1. Thiscauses the first and second brazing materials 13 and 14 to be melted sothat the molten second brazing material 14 flows into the vacuum sealingsection 15. The temperature of the assembly is maintained at the levelT1 for a predetermined time duration, to allow the molten second brazingmaterial 14 to become sufficiently intimate with the wall surfaces ofthe mouths of the respective inner and outer containers 1 and 8, withinthe vacuum sealing section 15.

Subsequently, while still maintaining the pressure within the chamber100 at the level P1, the heater 103 is deenergized and the table 102 islowered to its original position. Then, the assembly is moved into thesecond air lock chamber 100b which is now under the pressure of P1. Thiscauses the temperature of the assembly to be gradually lowered to alevel T2 which is lower than a solidifying point of the second brazingmaterial 14 but higher than the melting point of the first brazingmaterial 13. Consequently, the second brazing material 14 is solidifiedto seal the vacuum sealing section 15, so that the annular space 16 isformed into a vacuum insulating space 18.

After the second brazing material 14 has been sufficiently solidifiedand the brazing of the vacuum sealing section 15 has been completed, theinert gas is introduced, by opening the valve 109, into the second airlock chamber 100b to raise the pressure therein to a level P2 which isapproximate to the atmospheric pressure. This causes the heated andhence softened end walls 3 and 11 of the inner and outer containers 1and 8 to be deformed so as to be moved toward each other, so that theinterface between the end walls 3 and 11 is filled with the molten firstbrazing material 13 with a minimum of gap, and an excess brazingmaterial is forced out of the interface between the end walls 3 and 11toward the vacuum-heat-insulating space 18. Also, when the inert gas isintroduced into the second air lock chamber 100b, the temperature of theassembly is rapidly lowered to the normal temperature under theinfluence of the inert gas. As a result, the first brazing material 13closely filled in the interface between the end walls 3 and 11 issolidified with the lapse of time, so that the end walls 3 and 11 areclosely joined to each other into a unit. The thus processed assembly ofthe inner and outer containers 1 and 8 is then taken out from the secondair lock chamber 100b of the vacuum furnace 101.

According to the above-described steps, the vacuum-heat-insulatedcooking utensil is manufactured comprising the inner and outercontainers 1 and 8 which, as shown in FIGS. 5 to 7, have the annularvacuum-heat-insulating space 18 formed between the side walls 2 and 9,and have the end walls 3 and 11 intimately joined to each other to forma heat receiving bottom 20 of the cooking utensil. In the thusmanufactured vacuum-heat-insulated cooking utensil, as shown in FIG. 6,the end walls or bottoms 3 and 11 of the inner and outer containers 1and 8 are closely joined to each other by the first brazing material 13.Finally, as shown in FIG. 7, the downwardly extending outer peripheralportion 6 of the flange 5 which is shown by the phantom line in FIG. 7at the mouths of the respective inner and outer containers 1 and 8, isbent radially inwardly and is pressed against the flange 12 by anappropriate press (not shown). Thus, all of the manufacturing steps arecompleted.

According to the above-described manufacturing method of thevacuum-heat-insulated cooking utensil, the forming of the vacuum heatinsulating space 18, the sealing of the vacuum sealing section 15 andthe forming of the heat receiving bottom 20 can be performedsubsequently and at once, so that the manufacturing is extremelyfacilitated. In addition, a great number of cooking utensils can bemanufactured at a time, so that it is possible to reduce themanufacturing cost. Moreover, since the brazing of the vacuum sealingsection 15 and the heat receiving bottom 20 is performed under vacuum, afear is minimized that voids are developed in the brazing, which hasconventionally often become an issue in the brazing within theatmosphere, so that there is provided a brazing superior in quality.Furthermore, since the end walls 3 and 11 of the inner and outercontainers 1 and 8 are pressed against each other from the outside andare intimately joined to each other in unit when the heat receivingbottom 20 is formed, the first brazing material 13 is spread between theend walls 3 and 11 and is well diffused and, therefore, it is possibleto further ensure the brazing.

Although the embodiment has been described as utilizing the firstbrazing material 13 in the form of a thin disc and the second brazingmaterial 14 in the form of a ring, the present invention should not belimited to this embodiment. Brazing material in the form of a foil orpaste, for example, may be used as the first brazing material 13, andbrazing material in the form of a wire, a rod, granule having a particlesize greater than the gap of the vacuum sealing section, paste or thelike, for example, may be used as the second brazing material 14.Furthermore, the brazing materials 13 and 14 should not be limited onlyto the silver brazing filler metal, but may be suitably selected,dependent upon the material of the containers, temperature condition andthe like, from a group consisting of manganese copper, phosphorouscopper, nickel, palladium, aluminum brazing filler metals and the likewhich contain no components relatively high in vapor pressure.

When the first brazing material 13 is in the form of paste, is selected,the surface of the end wall 3 (see FIG. 2) should be uniformly coatedwith the brazing material 13 before putting the outer container 8 on theinner container 1. Also, when the second brazing material 14 is in theform of powder or granules, the second brazing material 14 should beuniformly distributed over the upper surface of the enlarged diameterportion 4 (see FIG. 2); and when the second brazing material 14 is inthe form of paste, the upper surface of the enlarged diameter portion 4should be uniformly coated with the second brazing material 14.

Additionally, although the above-described vacuum-heat-insulated cookingutensil has such a construction that the vacuum sealing section 15 isformed between the mouths of the respective inner and outer containers 1and 8, the present invention should not be limited to this construction.For example, the mouths of the respective inner and outer containers 1and 8 may previously be hermetically joined together by welding or thelike, and a vacuum sealing section similar in function to the sealingsection 15 may be provided in the side wall 2 and/or the side wall 9.The vacuum sealing section may be a hole formed through the side wall 2or 9 which is closed by the second brazing material. Even in this case,similar function and advantages can be obtained.

What is claimed is:
 1. A vacuum-heat-insulated cooking utensilcomprising:a pair of inner and outer containers, each of the containersbeing made of metal and having (i) a tubular side wall having first andsecond ends, and (ii) an end wall closing the first end of said sidewall, and having upper and lower surfaces, the side wall of said outercontainer having an inner diameter greater than an outer diameter of theside wall of said inner container. the entire area of the bottom surfaceof the end wall of said inner container being permanently and rigidlybrazed by a first brazing material to the upper surface of the end wallof said outer container to permanently connect the end walls together inintimate heat transfer relation and in such a manner that said innercontainer is disposed in coaxial relation to said outer container toform an annular space between the side walls of said inner and outercontainers, the second ends of the tubular side walls of said inner andouter containers being hermetically brazed by a second brazing materialto each other to close said annular space, said closed annular spacebeing evacuated to provide a heat-insulating space, said first brazingmaterial having a lower melting point than said second brazing material.2. A vacuum-heat-insulated cooking utensil according to claim 1, whereinthe second ends of the side walls of said inner and outer containers areformed respectively into first and second peripheral flanges directedradially outwardly, said first and second flanges being hermeticallybrazed to each other by said second brazing material to close saidannular space.
 3. A vacuum-heat-insulated cooking utensil according toclaim 2, wherein said side wall of said inner container includes anenlarged diameter portion adjacent to said second end of the side wallof said inner container, the outer diameter of said enlarged diameterportion being slightly less than the inner diameter of the side wall ofsaid outer container, an outer surface of said enlarged diameter portionbeing brazed to said side wall of said outer container.
 4. Avacuum-heat-insulated cooking utensil according to claim 3, wherein theouter end of said first flange is turned inward to enclose the outer endof said second flange.
 5. A vacuum-heat-insulated cooking utensilaccording to claim 1, wherein the first brazing material has a greaterthermal conductivity than the second brazing material.
 6. Avacuum-heat-insulated cooking utensil according to claim 1, wherein theend wall of each container extends substantially planar completelyacross the first end of the sidewall of the container.